This invention relates to an Invar alloy for use in a shadow mask and a shadow mask employing the same, and, more particularly, to an Invar alloy for use in a shadow mask and of the composition suited for forming a perforated matrix by etching.
The shadow mask that has a great number of perforations made in the form of a matrix on the basis of a given design standard is provided right in front of the fluorescent face of a color picture tube. Electron beams shot from electron guns positioned rearward pass through these perforations and are irradiated on fluorescent dots at given position on the fluorescent face to reproduce a color image thereon. At this time, however, it does not follow that all of the shot electron beams pass through the perforations, and a part of them directly bombards with the shadow mask to heat the shadow mask. As a result, the shadow mask may expand, so that the position of the perforations may get out of the designed standard position and be displaced to cause sometimes the phenomenon of color deviation.
For this reason, it has been conventionally practiced that the shadow mask is supported with a bimetal so that the whole of the shadow mask may be brought close to the fluorescent face with utilization of the bending of the bimetal when heated.
Recent years, it is also attempted to constitute the shadow mask itself with use of an Invar alloy which is a Ni-Fe base alloy typified by a 36% Ni-Fe alloy and endowed with a low thermal expansion characteristic, to prevent the phenomenon of color deviation.
In producing the shadow mask from this Invar alloy, a lump of Invar alloy of given composition is casted, for example, in a cast iron mold, and then forged and rolled to an original sheet having a given thickness. The thickness usually ranges from 0.1 to 0.3 mm. Subsequently, this original sheet is etched with use of, for example, a ferric chloride solution on the position to be perforated, to make perforations of given shape in the form of dots or a reed screen, followed by pressing to set its whole shape, and thereafter the whole thereof is oxidized, for example, with steam to blacken its surface for the purposes of improving heat radiation ability and preventing irregular reflection.
On the other hand, the Invar alloy used as material for the original sheet include various types conventionally known, but, in general, include those in which non-metallic inclusions typified by oxides, carbides and sulfides such as MnO, SiO.sub.2, MgO, Al.sub.2 O.sub.3, MnS and TiC, which may give influence to etching, are uniformly dispersed in alloy matrices such as Ni and Fe in the form of particles.
When the etching is effected on the original sheet of the Invar alloy having the alloy composition mentioned above, microscopic observation of the perforations thus bored can reveal a state such that the shape of the perforations is not in conformity with the shape pursuant to the design standard and thus the perforations are irregularly deformed or irregularities appear on inner wall faces of the perforations.
If the perforations are formed in such a state, it may follow that the electron beams shot from electron guns are not irradiated on the fluorescent face in an appropriate state because the course of irradiation is hindered by the irregularity of the shape of perforations and the state of the inner walls, causing the phenomenon of color deviation in short.
In addition to the problem of the non-uniformity of the inner walls of the perforations, the perforations formed by etching may have a problem that, as shown in FIG. 1, they have a large inner diameter at the surface part of the original sheet and on the other hand have a small inner diameter at the thicknesswise central part of the original sheet, and thus it is required to make the inner diameter as uniform as possible in the range of from the surface part to thicknesswise central part of the original sheet, preferably to make the inner diameter (R) at the surface part within 4-fold of the inner diameter (r) at the thicknesswise central part. In FIG. 1, 1 is an original sheet and 2 is a perforation